Attention to particular stimuli greatly improves performance that depends on those stimuli, while degrading performance on other stimuli. Neurophysiological studies have shown that attention changes the responses of neurons in visual cerebral cortex, but many questions remain about the neuronal mechanisms through which attention alters behavior. The proposed experiments will address two specific questions about how spatial attention affects visual processing in the visual cortex of monkeys. The first specific aim will examine how attention affects the responses of individual neurons in visual cortex. Many studies have shown that attending to a stimulus enhances the responses of neurons that represent that stimulus, but few have examined the form of this enhancement. Measurement of the effects of attention on responses to stimuli of different orientations and different contrasts have led to different views about whether attention acts by uniformly increasing the strength of neuronal responses to all stimuli. Experiments of the first specific aim will resolve this discrepancy by examining interactions among attention, orientation and contrast in determining the responses of individual neurons. Additionally, they will examine about how attention affects the timing of visual responses. The second specific aim will examine how attention affects the relationship between neuronal responses and behavior. It has been observed that the ability of individual neurons to discriminate stimuli can approach or match the performance of the subject, suggesting a close link between neuronal and behavioral performance. Recent results show that attention alters this link for some neurons, but leave open the possibility that a close relationship between neuronal and behavioral performance persists across attentional states for those neurons that are best suited for current task. The second specific aim will test this possibility by examining how attention affects the relationship between neuronal and behavioral performance for neurons during the performance of different visual tasks. The results from these experiments will greatly extend our understanding of how attention changes visual representations in cerebral cortex and improves behavioral performance, and will provide new insight about how individual neurons contribute to visual behaviors.